/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.reactive.resource;

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.net.URI;
import java.net.URL;
import java.util.List;

import reactor.core.publisher.Mono;

import org.springframework.core.io.AbstractResource;
import org.springframework.core.io.Resource;
import org.springframework.http.HttpHeaders;
import org.springframework.lang.Nullable;
import org.springframework.web.server.ServerWebExchange;

/**
 * A {@code ResourceResolver} that delegates to the chain to locate a resource
 * and then attempts to find a variation with the ".gz" extension.
 *
 * <p>The resolver gets involved only if the "Accept-Encoding" request header
 * contains the value "gzip" indicating the client accepts gzipped responses.
 *
 * @author Rossen Stoyanchev
 * @since 5.0
 */
public class GzipResourceResolver extends AbstractResourceResolver {

    @Override
    protected Mono<Resource> resolveResourceInternal(@Nullable ServerWebExchange exchange,
            String requestPath, List<? extends Resource> locations, ResourceResolverChain chain) {

        return chain.resolveResource(exchange, requestPath, locations)
                .map(resource -> {
                    if (exchange == null || isGzipAccepted(exchange)) {
                        try {
                            Resource gzipped = new GzippedResource(resource);
                            if (gzipped.exists()) {
                                resource = gzipped;
                            }
                        } catch (IOException ex) {
                            logger.trace("No gzipped resource for [" + resource.getFilename() + "]", ex);
                        }
                    }
                    return resource;
                });
    }

    private boolean isGzipAccepted(ServerWebExchange exchange) {
        String value = exchange.getRequest().getHeaders().getFirst("Accept-Encoding");
        return (value != null && value.toLowerCase().contains("gzip"));
    }

    @Override
    protected Mono<String> resolveUrlPathInternal(String resourceUrlPath,
            List<? extends Resource> locations, ResourceResolverChain chain) {

        return chain.resolveUrlPath(resourceUrlPath, locations);
    }


    static final class GzippedResource extends AbstractResource implements HttpResource {

        private final Resource original;

        private final Resource gzipped;

        public GzippedResource(Resource original) throws IOException {
            this.original = original;
            this.gzipped = original.createRelative(original.getFilename() + ".gz");
        }

        @Override
        public InputStream getInputStream() throws IOException {
            return this.gzipped.getInputStream();
        }

        @Override
        public boolean exists() {
            return this.gzipped.exists();
        }

        @Override
        public boolean isReadable() {
            return this.gzipped.isReadable();
        }

        @Override
        public boolean isOpen() {
            return this.gzipped.isOpen();
        }

        @Override
        public boolean isFile() {
            return this.gzipped.isFile();
        }

        @Override
        public URL getURL() throws IOException {
            return this.gzipped.getURL();
        }

        @Override
        public URI getURI() throws IOException {
            return this.gzipped.getURI();
        }

        @Override
        public File getFile() throws IOException {
            return this.gzipped.getFile();
        }

        @Override
        public long contentLength() throws IOException {
            return this.gzipped.contentLength();
        }

        @Override
        public long lastModified() throws IOException {
            return this.gzipped.lastModified();
        }

        @Override
        public Resource createRelative(String relativePath) throws IOException {
            return this.gzipped.createRelative(relativePath);
        }

        @Override
        @Nullable
        public String getFilename() {
            return this.original.getFilename();
        }

        @Override
        public String getDescription() {
            return this.gzipped.getDescription();
        }

        @Override
        public HttpHeaders getResponseHeaders() {
            HttpHeaders headers;
            if (this.original instanceof HttpResource) {
                headers = ((HttpResource) this.original).getResponseHeaders();
            } else {
                headers = new HttpHeaders();
            }
            headers.add(HttpHeaders.CONTENT_ENCODING, "gzip");
            return headers;
        }
    }

}
